net: ptp: Initial PTP library implementation

This is an initial commit that introduces PTP library in the Zephyr.
It adds basic Kconfig symbols for enabling library, CMakeLists.txt
for compiling it, public header file, and initial PTP thread definition
and library initialization. Implementation of the functional PTP stack
will be introduced in following commits.

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>